1. Field of the Invention
The present invention relates to a color filter substrate structure of a liquid crystal display device, and more particularly, to a method of fabricating a color filter substrate having a spacer directly contacting a substrate.
2. Description of the Related Art
Presently, liquid crystal display (LCD) devices are commonly used in various fields from image display devices, such as computer monitors, TV receivers, and portable telephones, to information display panels used for automobiles and airplanes, for example.
FIG. 1 is a perspective view of an upper substrate and a lower substrate of an LCD device according to the related art. In FIG. 1, an LCD device includes a transparent substrate 21, a color filter layer 8 including a black matrix 6 and a sub-color filter layer 7 of red, green, and blue on the transparent substrate 21, a color filter substrate 1 having a transparent common electrode 18 on the color filter layer 8, a TFT array substrate 2 including a plurality of switching devices T arranged on the substrate 3 in a matrix configuration, and a pixel electrode 5 corresponding to the respective switching devices T. In addition, a liquid crystal material 10 is injected between the color filter substrate 1 and the TFT array substrate 2.
A plurality of gate lines 11 arranged in rows and a plurality of data lines 12 arranged in columns are on formed on the TFT array substrate 2, such that the gate and data lines intersect and cross each other. In addition, a thin film transistor T corresponding to the switching device is formed at the intersection points between the gate lines and the data lines in a matrix configuration.
A pixel region 4 is formed on the TFT array substrate 2 corresponding to the gate lines 11 and the data lines 12, and the pixel electrode 5 is formed on the pixel region 4. The pixel electrode 5 is formed of a transparent electrode material having excellent light transmittance, such as Indium Tin Oxide (ITO).
FIG. 2 is a flow chart for a fabrication process of an LCD device according to the related art. In FIG. 2, the fabrication process includes steps of: preparing a lower (TFT) substrate; forming an alignment layer on the lower substrate; rubbing the alignment layer for alignment of liquid crystal material (not shown); forming a seal pattern; scattering spacers (not shown); bonding an upper substrate and the lower substrate together; cutting the bonded substrates into a plurality of unit cells; and injecting the liquid crystal material into the unit cell.
During the preparation of the lower substrate, a plurality of the gate lines horizontally arranged and a plurality of the data lines vertically arranged by crossing the gate lines are formed on the lower substrate. Then, a plurality of the thin film transistors are formed in a matrix configuration at the intersection points of the gate lines and the data lines. Next, the pixel electrode corresponding to the switching device T is formed on the TFT array substrate.
During the formation of the alignment layer, a thin film of polymer material is uniformly coated on the lower substrate. It is essential that the alignment layer is uniformly formed so that the subsequent process of rubbing the alignment layer may be uniformly performed and alignment of the liquid crystal material may be uniform.
During the process of rubbing the alignment layer, a cloth is used to rub the alignment layer along a constant direction to uniformly align the liquid crystal material. The rubbing process is important to set the initial alignment direction of the liquid crystal material. When the rubbing process is properly performed, normal driving of the liquid crystal material is possible to provide uniform driving characteristics of the LCD device. Generally, the alignment layer is formed of polyimide-based organic materials.
The seal pattern forms a gap between the upper substrate and the lower substrate for the liquid crystal material injection, and prevents the injected liquid crystal material from leaking out from the bonded substrates. Accordingly, during the formation process of the seal pattern, a constant pattern is formed along a periphery of an active region of the lower substrate using a thermosetting resin during a screen printing method.
During the process of scattering the spacers, spacers of a predetermined size are used in order to maintain a constant cell gap between the upper and lower substrates. Accordingly, the spacers have to be scattered uniformly on the TFT array substrate. Scattering methods of the spacers include a wet scattering method for scattering the spacers within an alcohol carrier solution, and a dry scattering method for scattering only the spacers without using a carrier solution. In addition, the dry scattering method includes an electrostatic scattering method using static electricity, and a non-electrostatic scattering method using gas pressure. The non-electrostatic scattering method is commonly used in the liquid crystal cell structure having a low static electricity resistance.
During the bonding process, the upper and lower substrates are bonded together along the seal pattern. The bonding process is determined by a predetermined margin between the upper and lower substrates, which may be several microns. If the bonded substrates exceed the predetermined margin, light may leak from the liquid crystal display device, thereby reducing the picture quality of driving the liquid crystal display device.
Next, the bonded substrates are divided into a plurality of unit cells by cutting the upper and/or lower substrates. In previous processes for cutting the liquid crystal cells, the liquid crystal material was simultaneously injected into several unit cells and the cells were cut as a unit cell. However, as the area of the LCD devices increased, the liquid crystal cells were first cut into the unit cells and then injected with the liquid crystal material.
Finally, the liquid crystal material is injected into the cut unit liquid crystal cells. The unit liquid crystal cell has a cell gap of several microns and an area of several hundreds of square centimeters. Accordingly, a vacuum injection method for injecting the liquid crystal material into the unit liquid crystal cells using a pressure difference between interior and exterior regions of the liquid crystal cells is commonly used.
FIGS. 3A to 3F are cross sectional views of a fabrication process of an upper substrate of an LCD device according to the related art. In FIG. 3A, a black matrix 32 is formed on a transparent glass substrate 31 at a region of an upper substrate that corresponds to gate and data lines and switching devices of the lower substrate. Generally, the black matrix 32 is formed between sub-color filter of red, green, and blue, and blocks light that passes through a reverse tilt domain formed at a peripheral portion of a pixel electrode of the lower substrate. A material for forming the black matrix 32 includes a metal thin film, such as Cr, having an optical density more than 3.5, or an organic material. In addition, a double layer, such as Cr/CrOx, may be used for low reflection. Accordingly, a proper material of the black matrix may be selected for a desired purpose.
In FIG. 3B, a color filter is formed using a pigment scattering method. However, several different methods may be used, such as dyeing, depositing, and printing. First, a red color resin is deposited on the substrate 31 upon which the black matrix 32 has been previously formed. Then, the red color resin is selectively exposed to light, thereby forming a red sub color filter 33a at a desired region.
Next, a green color resin is deposited on the substrate upon which the red sub-color filter has been previously formed, and selectively exposed to light, thereby patterning the green sub-color filter 33b at a desired region. Likewise, a blue sub-color filter 33c is formed by repeating the same process.
In FIG. 3C, a process for forming a overcoat layer 34 includes forming a transparent resin having an insulating characteristic on the substrate 31 upon which the sub-color filters 33a, 33b, and 33c have been previously formed, thereby forming an overcoat layer 34. The overcoat layer 34 is not necessarily formed and may be omitted. When a Cr-based metal is used as the black matrix 32, the overcoat layer 34 is not needed since a thickness of the black matrix 32 is as thin as a few thousands of angstroms. In addition, when a resin-based black matrix 32 is used, a thickness may be 1 to 1.5 μm, thereby requiring the overcoat layer 34.
In FIG. 3D, a common electrode 35 is formed on the color filter substrate upon which the overcoat layer 34 has been previously formed. The common electrode is a transparent electrode and is commonly formed of indium tin oxide (ITO). A common voltage is supplied to the common electrode, so that an electric field is formed in combination with a pixel voltage applied to the pixel electrode on the array substrate, thereby driving the liquid crystal material.
In FIG. 3E, spacers 36 are formed in which a transparent organic film is formed on the substrate 31 upon which the common electrode 35 has been previously formed, and the spacers 36 are patterned with a specific shape by photolithographic and an etching processes. Both shape and height of the spacers 36 are determined by the photolithographic process that includes a chemical reaction by irradiation of light and cross linking. Accordingly, the construction of the spacers 36 is changed into a net structure and the spacers 36 are resistant to etching. Thus, the spacers 36 may be formed having a more minute pattern, thereby increasing the mechanical strength of the spacers 36. Increasing the mechanical strength of the spacers 36 results in increasing mechanical deformation of the spacers. For example, in in-plane switching (IPS) mode LCD devices, an electric field is formed on the array substrate 31 and the metal-based black matrix may influence the electric field when the liquid crystal material is driven. Accordingly, carbon-based resins are used as the material of the black matrix 32. However, the carbon-based black matrix resins have a mechanical strength lower than that of the acryl-based color resin and the overcoat layer 34 so that the black matrix resin 32 is mechanically deformed by external impact to the spacers 36. In addition, the spacers 36 can be formed on the overcoat layer 34 prior to formation of the common electrode 35. However, the same problems regarding mechanic deformation may be generated.
In FIG. 3F, a process for forming the alignment layer 37 includes forming an organic insulating film, such as polyimide, on the color filter substrate upon which the spacers have been previously formed. Then, a rubbing process is performed for forming a predetermined groove on the alignment layer 37 in order to provide an alignment of the liquid crystal material, thereby completing the upper substrate of the LCD device.
It is possible that the spacers 36 can be formed after forming the alignment layer 37. However, the alignment layer 37 is chemically damaged during the etching process. Thus, the alignment layer 37 is commonly formed after formation of the spacers 36.
However, during fabrication of the upper substrate of the LCD device and fabrication of the spacers 36, the spacers 36 are formed at a region corresponding to the black matrix 32. When external pressure is applied to the spacers 36, the carbon resin-based black matrix 32 corresponding to bottom portions of the spacers 36 are mechanically deformed such that the spacers 36 are deformed along a direction towards the black matrix 32. This is a significant problem in IPS-mode LCD devices that use the carbon resin-based black matrix. If the spacers are deformed, the cell gap is not uniformly maintained. Accordingly, differences in displayed image brightness are generated, thereby creating smear on the image screen. This phenomenon is also generated where the spacers 36 are formed at a color filter region of the upper substrate corresponding to the pixel region of the TFT array substrate of the LCD device.